Thermoelectricity



A. EI STEIN ET AL May 12, 1964 THERMOELECTRICITY Filed Dec. 7, 1959 INV EN TOR:

ARNOLD EPSTEIN STANLEY M. KULIFAY ATTORNEY i Specific preferred compositions centageby eight: q

THERMOELECTRICITY Arnold Epstein and Stanley M. KulifayfDayton, Ohio,

assignors to Monsanto Chemical Mo.,* a corporation of Delaware Filed Dec. 7, 1959, Ser. No. 857,891

7 Claims. "(CL 62-3) Company, St. Louis,

f -Th ef' present invention relates to thermoelectric com positionswhich' are of'ut'ilit'y fdFt'hedirectcoiiversion of electricity either" cooling or heatingproc essesfand also the gen'ratioii or electricity by the application of heat to of the said bodies in devices such as thermocouples, thermistors, radiation detectors, pyrometers, and thermorelays.

Y l tmqe es r m r als. con lat s athe P V le v atien re s n e is atsd te m of ashvid P i s te g m qs qn. isihsn s me! t l i and 3,132,488 Patented May 12, 1064 Ice sion and dispersion" methods, e.g. grinding and pulverizing, or by chemical: precipitation methods-.sucli-asby precipitation or coprecipitation of the finelydivided binary and higher compositions from solutions of the desired components precipitated by hydrazine and ,other p c p ta n a en 's' I f i I The size of the particles canbe' varied (we e wide range andit has been found that exceedingly finely-divided particles are'effective in'the presentrelationship lt is preferred that the particles be of less than,2,500 micron diameter. However, very finely-divided'particles;such as selenium. Theconsolidation of the discrete particles is effected by various pressing operations The particles, of the powder are-obtained by chemical coprecipitationor froma fused, e.g., densjiform solid by grinding, commihution or abrasion.- Su'ch mechanical subdivision to chtain-the powder is carried out witha pressed or Impressed i solid-state" eaction mixture of the individual component elements, or amixture of powders. I 1 V The major cornponent of the product is 'silven'the cop .per arid tellfirium must be presentto the extent of at least 0'i005-% by weight while the selenium is the component added to bring the formulations into the range defined below. Sulfur is an optional component, and when present is' preferably employed in the 'ran'ge" of 0 to" 8% by weight.- Various chemical compounds and also new I Ts'toichiometrie formsiri'ay th'us e'x'ist'ih the produce The broad range of compositions, in percent by weight,

0.005 to 2 Tellurium l ,Selenium 4.000 to 29.99

are given below in per- Cop'perTi.

Sil'

: ver..

Tellurium I "The compounds are used in both stoic hiometric non-st-oichiometriciorm, and also modified or doped by 'the addition thereto of an agent selected from-the group-consisting of sulfur, silVer selenium, telluriurn,

' copper,.- nickel;. mercury, le-ad, cadmium, bis muth, anti? ewtha li mt d-ma n se.- Ihe p t o l i i iadd y incl d em o one binarysor,

material as the dopant or additive is-broadly in the range Percent; V 70 to 85 50 micron particles.-and. even smaller for example 10 micron diameter particles have been found to operate very efficiently in the present. method. {A still more preferred particle size'range. is f om 0.01 to 1,000'microns.

The formation ofltherm'oelectnic bodies in accordance with the present invention is carried outby consolidating the discrete particles by subjecting them to a pressure of at least 500 pounds per square inch, .or preferably at least 1,000 pounds pensquare'inchr l-Iigher pressures may be used as desired and pressures of the order of 2,500 to 200,000 pounds P?! quar inch have been found useful for this purpose. A suitable temperature range is from a temperature of 30 C. to a temperature of'50" C. below the melting point, while a preferred temperature range is from 50 C. to 600 C. A still more preferred temperature range is from 75 C. to; 500". C. The us'eof higher pressures makes it possible to operatewith lower temperatures. It is desirable, ;though not necessary, to employ-'a vacuum of 10" to '10- mm. Hg during theheabting step. xIn the'practice of. the present invention, heating times of 5 minutes to 24 hours', or preferably, of 15 minutes to 2 hours are effective, in a vacuum of 10- to 10* film. He. 7

the individual particles into a. shaped mass of superior thermoelectric properties for use in various specific applications. In a preferredembodiment of the invention, this pressed piece is heated as described above to further imheating may; be combined in a so-called hot pressing vhigher chemical:composition added to the above basic a .of less than 15 by W'eight, or preferably from 0.005 to -.,1'5% by weight. A still'more preferred range, particu-- larly inithe case of single elements as the-additive is from 0.Q1 f7 to1;0% by weight relative to the weiglitofthe base material. i

'ffoper-ationin which la; heatedndie is employed to provide both pressure and the desiredtemperature as a unitary 'pr oces sf m .It has been found that the products thus obtained are characteriied by an ultimate structure based upon the original particulate form *anddilfer radically from a cast e.g. densiform or melted "type of product. Thus in the rr'ieasurementof thethermoelectric'power. of the compositionsuof thepresent invention, ithas been foundthat far superiorresults are obtained by'theemployme'nt. of particulate .starting' materials subjected to elevated pressure and moderate temperatures incontrast to the same mate rials melted to aliquidstate and allowed tosolidify. such as by casting, e'xtruding and other melt processes.

The mechanism by which modificatiomof the therm o electric power is obtained by doping has not been completely elucidated. r However, it hasbeen found that the v range of 10 1 to 10 :carriers', per. cc., that is-Lfrom 0.000001% to 0.001% by weight, of additives or dopants characteristic of typical semi-conductor compositions, e.g., in'transistors, and rectifiers and diodes are notefiectivein thepresentthermoelectriccompositions? ,I

It has been round that when the above bas iifmatrix compositions" and the niodified'fderivatives thereof are 1, used in a powdered rompa ngnian as'particle's ofless than 2,500 microns diameter and preferably in the par- .,;T he present invention iscarried out employing discrete 7 particles of the desired'comvposition asrthe starting material. Such finelydivided or powdered materials are obtained by various methods, 'suc'h' as' by mechanical'abraticle size range of from 0.01 to 1,000'microns diameter,

and subje cted to a pressing step, the resultant product is,"

improved. Additional heat'treatment, or sintering, gives,

even further improvement. The improvement "is shown The present process is based upon the consolidation of ansaaes in the gain of thermoelectric utility over the prior art compositions as revealed by the so-called figure of merit Z, defined as the ratio of the Seebeck coefiicient or thermoelectric power, S, squared to the product of the electrical resistivity, p, and thermal conductivity, K.

(Semiconductor Thermoelements and Thermoelectric Cooling, p. l, A. F. Ioffe, Infosearch Limited, London, 1957 V V Thermoelectric generation-for maximum efliciency T hot temp.

T Temp. at cold end T T1+ o o w )Z+1+ T1 Part 1, Chap. 2,.p. 40, A. F. Ioffe For thermoelectric cooling the maximum theoretical coefiicient of performance, E0, is related to Z as follows:

e =Coefficient of performance T =the warmer, and

T +the cooler junction temperature Part 2, Chap. I, Ioife pp. 99, 115

The following examples illustrate specific embodiments of the invention:

' components being present in an aqueous system. The 1 4 EXAMPLE 2 A similar composition without the sulfur but in which all the other components were proportionally increased was also prepared by wet precipitation using ammoniacal hydrazine hydrochloride as the precipitating component with soluble compounds of the first four of the above sulfur-free, non-stoichiome'tric compound had the following composition:

The finely powdered products of compositions I, II, and III were subjected to electrical measurement in the form of pressed, sintered and unsintered powders, i.e., particle diameters of 0.1 to 250 microns (avg. particle size: 10 microns. An earlier worker using composition III in a fused (densiform) state obtained figures of merit of about 1 X 10*. The results for the above three compositions, all of which gave N-type conductivity were as follows:

1 II III Unsintered Sintered Unslntered Sintered Unsintered Sintered.

Electricalreslstivlty(ohm-em.). 1.3 10- 1.34X10- 6.95X10- 3. 52x10 12. 6 10- 12.6)(10- Thermoelectricpower(uv.ldeg.) 53 I 80 38 55 194 233 Thermal conductivity, K (eaL/ cm. sec. deg.) 2. 01 10- 2. 09 10- 3. 55X10- 5. 50 10- 7. 05X10- 3. 43 10- Conductivity type N N a N N- N N Figure of merit Z, X10 (tem H No'rE.A1l slntcring experiments were conducted at 200 C. for 15-30 minutes at a pressure oi10' 1()- mm.

EXAMPLE 1 The prior art has recognized that fused (completely melted and re-solidified) complex compositions such as a composition of copper, silver, tellurium, selenium, and

sulfur may be used as thermoelectric elements. However, the applicants have found that the present materials in pressedand sintered powder form give greatly superior figures of merit over the prior art compositions. In order toshow this effect, compositions were prepared by wet precipitation using ammoniacal hydrazine hydrochloride in aqueous solution mixed with aqueous solutions of com pounds of the following elements:

a In this formulation, I, the percent of copper and of silver were not altered from those given in composition III below. However, the sulfur was omitted and replaced by increases in the tellurium and seleniumonly, in proportion to the amounts called for in composition III.

While the exact mechanism of the physical and chemical behavior of the present thermoelectric bodies has not been completely elucidated, it would appear that the product thus obtained differs from conventional densiform thermoelectric materials (such as bismuth telluride) and methods. of preparation thereof in the following ways:

, (1) The material'as prepared in the powder form by the precipitation method lends itself easily to a (crystal) disordering process but yet retains chemical and thermodynamic stability within the framework of utility.

' (2) It is usually found that compressed or compacted powders exhibit electrical resistivities which may be much poorer than those found for'the same material in the densiform structure, such'as by melting and casting. However, the present invention, in compressing and compacting powders, exhibits surprisingly desirable resistivities and other electrical properties (of thermoelectric utility). The present invention makes it possible to achieve an unforeseen improvement in thermal conductivity, K, even though the commonly-used ratio of the thermoelectric power squared to the electrical resistivity (S p) for the densiform materialand compacted powder are comparable. We note this especially in the case of a simple material, e. g., silver selenide where the electrical and thermal properties of (a) a sample which had been melted and i then solidified and (b) a compacted pQWder sample, are

T56 present invention includes doped or modified compositions which are employed in a consolidated form, e.g.,

pressed, sintered particles, to give superior thermoelectric materials. For example, the above'formulation III is distinguishablefover compositi'onsI and II by the addition of sulfur as a dop'a'ht. "The'densiform, i.e'., fused and solidified form of composition IIIhas been reported to have a figure of merit of about 1 '10 However, when this samecomposition, III, is produced as a particulate solid of 1 to 250 microns particle size and consolidatedlby pressing at 20,000 psi. and thensintered at 200 C. for 30 minutes and at 10* mini Hg, the figure of meritis 3' 10 v a I (3) An example of the advantage of doping together with compacting powder may be seen in the high figures i of merit that can be obtained in comparison to the melted,

densiform, stoichiometric material. The modified material ;-is a pressed, doped (0.1%v Se) Ag Se which is subjected to (a) vpressing Without sintering, and (b) pressing .with sintering in vacuum of 10 to 10 mm. Hg. (Sinteriug can also be done. using-other conventional methods s chasiinertgas blanketing under vacuum, atmospheric,

.or super-atmospheric pressures.)

5 Norm-Above properties measured at 27 C.

Anexample in which high figures of merit can be ob tained 'without sintering is Ag Se-l-0.1% copperr The pertinent electrical and thermal properties are reviewedv I below. The figure of merit calculated from the experi-' 7 mental data was 8X l at room temperature.

g'jo(chill-(311.) 1.46 l O- Q S(; /.v./deg.) 76

K (cal./cm.see.deg.) 1.l7 -10 Conductivity type 'Z 8.1 10" (4) It will be noted that the above is a doped material. The attainment of the high figure of merit has been found to be a consequence of the addition of a dopantwhich 'may create a certain disordering quality in the material or a gradient (variation of concentration of dopant from one end of sample to other) such that the particular elec- 'trical and thermal properties are found, with the aid of sintering, generally, to improve and give rise to particcompared. 1 ularly desirable thermoelectric features.

Ag zSe AgaSe+g AgQSe-Ff AgrSe 5 0.01% Se 0.1%.Se

Melted Pressed Electrical resistivity (Kl-cm) 6.8)(- s;6s 10- '7.45 10- I I and 7 powder S m/deg.) 5 I 115 117 solidified K (ca1.lem. sec. deg 1.49 x 10- 1.39 x 10- 1.24 10- Conductivity type L N a N I v I Z 2.1 x10- 3.4.x 10- 3.5x 10- Electrical resistivity, p, ohm-cm 8.5)(10- 3.75x10- 1O I I Seebeckcoeflicient, S, rm/deg ..I 120 74 T S /p '-hmi t K I d- (IS8X1O-Z igxiO-g Norn.Temperature=27 C. The! con ue vi .cm.sec. e 2X10- X 0-. I Conductivitytype. Y; N N The advantages of the present material combining prepl f m C 115x10 aration and processing are that they (1) permit the attainment of electrical or thermal properties either com- I parable or superior to those obtained by fusion techniques,

. introduce a method of controlling thermal-conductivity I or almost independently of electrical resistivity.

In order to obtain the finely divided particles of the present compositions as well as the mixtures and additivemodified compositions described above, a preferred method is chemical coprecipitation For example, if'it is desired to prepare composition II, solutions of the pre-mixed requisite amounts of a silver salt, a copper salt, a selenium compound such as selenous acid, and a tellurium com I pound may be precipitated from an aqueous solution. In the same way, an excess of any component may be em- .ployed; for example, 0.1% by weight excess of silver tion may be carried out by precipi tating the desired pro 7 portions of mercury andisilver salts, a copper compound anda selenium and tellurium source. This'method is-also applicable to the addition of bismuth as an additive in combination with the compositions, as described below, and withother elemental materials described as doping additives, In the final consolidated particulate compositions various proportions of 'dopants may be disso1ved in the space lattice of the base composition or maybe con-, centrated at the grain .boundaries of the'ultimate consoli dated product obtained by the application of'pressure and heat as described above. However, the applicants are not "bound by this mechanism or the other mechanisms described hereinsince other physical combinations may also exist in thisv system. V I I I EXAMPLE 4 V V The preparation of composition I, above, was carried 'out' as follows:

One-quarter gm. copper and 1935000 gm. silver were dissolved together in' 75 ml. 1:1 nitric acid. In a-separate container, 0.2310 gm. tellurium was dissolved in 1-7 ml. 1

boiling, and the mixed metals solution above added thereto with constant swirling, precipitating the black product.

This was boiled for minutes, allowed to settle, and the heavy product thoroughly washed by decantation with wa; ter and finally methanol. It was dried under vacuum at 70 c. I I I I I EXAMPLE 5 The preparation of composition II described above was carried out as follows. A weight of 0.2702 gm. copper and 21.0567 gm. silver were dissolved together in 80 m1. 1:1 nitric acid. In a separate container 0.1621 gm. tellurium was dissolved in 17 ml. 1:1 Aqua Regia. In a third container was dissolved 3.5110 gm. selenium in 40 ml. 2:1 nitric acid. Now 100 ml. ammonium hydroxide was added to the silver-copper solution, with 50 ml. being also added to each of the tellurium and selenium solutions. All three solutions were thoroughly mixed.

The rest of the procedure as to precipitation was the same as for Example 4.

EXAMPLE 6 7 The preparation of composition III, above, was carried V out as follows:

A weight of 0.2500 gm. copper and 19.5000 gm. silver were dissolved together in 80 ml. 1:1 nitric acid. In separate containers were dissolved 3.2500 gm. selenium in 40 ml. 2:1 nitric acid, and 0.1500 gm. telluriurn in 17 ml. 1:1 Aqua Regia. Now 100 ml. ammonium hydroxide was added to thev Cu-Ag solution, with 50 ml. being added to each of the Te and Se solutions. The three solutions were thoroughly mixed.

The reducing solution was made as described in Examples I and II- with the addition, however, of 4.38 gm. thioacetamide of 98.8% assay, to provide a source of sulfur as sulfide ion upon heating. The mixed metals solution was added to this boiling reducing solution as previously described. The rest of the procedure was as described for Example 4.

The drawing of the present patent application shows a specific embodiment of the invention as a thermoelectric device. The apparatus shown in the present drawing may be used for the production of cold or heated atmospheres by the application of a direct current. Various compositions contemplated within thepresent invention may be either N-type or P-type. V

Referring to the drawing, 7 the thermoelectric device shown is composed of two thermoelectrically dilferent intermediate conductive part 3 'of negligible thermoelecsneaaes tor. An energizing circuit comprising a direct current source 10, aresistor 9, and a control switch 11 is connected'to the element by copper end terminals 4 and 5.-

I The end terminals are provided with single turn pipe coils 6 and 8 through'which a heat transporting fluid may be pumped to maintain them at a relatively constant temperature. Thus, when the action of the current through the thermoelectric junction produces a temperaturediffer- 'ential between the intermediate terminal 3 and the end terminals 4 and 5, the end terminals may be maintained at a constant temperature and the intermediate one may be reduced or increased in. temperature.

EXAMPLE 7 I In an arrangement as shown in the drawing of this patent application, the element 2 is' respectively cohsolidated, particulate, sintered composition II (84.23% Ag, 1.08% Cu, 0.65% Te, 14.04% Se, and 0% S) which has -a merit factor, Z=4 l0 Element 1 is bismuth telluride, Bi Te which has a merit factor, Z =2.2 10 v EXAMPLE 8 In the above assembly shown in. the drawing; e.g., element 2 is consolidated, particulate, sintered composition lll (78 wt. percent Ag, 1% Cu, 0.6% Te, 13% Se and 7.4% S), Z' 3 10 and element 1 is bismuth telluride, Bi Te Z=2.2 10 When this thermocouple pair is used as a device for thermoelectric power generation, it is found that the maintenance of a temperature of 100 C. on element 3 and 0 C. on elements 4 tric power. The N-type thermoelectric member 2 consists of one of theabove-described consolidated particulate compositions. 'The P-type composition "is bismuth telluride. Other thermoelectrically active P-type compositions may alsobe' used in this relationship. While it is desirable to use the present consolidated particulate forms ofmaterials for both the P-type and N-typemer'n'oers,

thermoelectric couples may alsobe formed in which one of the members is of the conventional densiform type, but

the. second member is a thermoelectric material of the above-described invention based upon the use of finely divided materials subjected to pressure-consolidation.

The member 2 consists of an N-type thermoelectric material, according to the invention. This material may be an alloy or compound such as bismuth tellurid'e, Bi Te with the use of an additive component such as a sulfide or selenide. I a

The intermediate part 3 which separates the members 1 and 2 to form a thermoelectric junction between them, consists preferably of a good conductor such as copper. This material serves as a cooling therminal to cool a space, or for the removal of heat from a medium' and may be contacted by a'pipe coil 7 to conduct a fluid coolant .to a-distant location. I a As used herein, the term space includes not only a gaseous or fluid volume but also solid objects and devices. An example of a gaseous space is that in a household refrigerator, while a device is a transistor or an infra-red detector. e

Alternatively, the member 3 may be shaped as a thin vane or other structure for cooling in its immediate en- 02 watts, e.g., 5' amperes at 43 millivolts.

As a further example of the'advantages of the present procedure utilizing finely divided powders, pressing, and sintering on the figure of merit, experiments were car: ried out using Ag Se+.75% Se in (a) powder form (b) densiform state (0) repowdered from densiform, in both the original state and in the sintered state. These are denoted in the'table below in columns A to F. It will be noted that quite unexpectedly the finely-divided powders pressed but unsintered (A) and the repowdered (finelydivided) and repressed (but unsintered) fused (densiform) material (D) both show about the same figure of volume as a commercial or household type of refrigeramerit (Z) at room temperature (T =25 C.) as one finds for the fused or melted Ag Se+0.75% Se. The figure of merit found was 1.3 lO Normally one would expect the powdered material to be vastly inferior. Further, on sintering at 200 C., it' is noted that the pressed powders (B, F) show much higher values of figure of vmerit than is found for the fused material (C, E). In

the case of the material which was repowdered (finely divided) and repressed from the fused (densiform) Ag Se-l-0.75% Se and then sintered, the improvement is quite marked.

It is noted that heat-treating the fused material itself at 200 C. for 15 minutes shows an improvement in fig- The merit figure increased from a me of merit also. value of 1.3 x 10* to 1.5 1O (D, E) everal unexpected effects then may be noted from these experiments: (1) finely divided powders (of Ag Se+0.75% Se) when pressed show aboutthe same figure of merit at T :25 C. as found for fused densiform materials which are normally regarded as showing superior-properties; (2) sintering under proper conditions can improve the figure of merit of pressed powders measureably over that found for fused (densiform) materials; (3) by powdering fused materials and sintering, radical for fused materials; (4) properly heat-treating fused materials can improve the figure of merit of fused materials.

a pressure of at least 1000 p.s.i. and heated at a temperature between 50 C. and 600 C. I j

6 A process of cooling which comprises applying a V Table Ag Se=0.75% Se I A. B O D E F Fused (but Fused, re- Powder Powder Fused matenot repowpowdered, pressed but pressed, Fused rial repowdered) and repressed unsiutered sintered (melted) d ered, resintered and sintered (200 C. for (densifcrm) pressed but (heat treated (200 C. for

30 min.) unsintered at 200 C. min.)

for 15 min.)

Resistivity, p (ohm-em.) 5. 98 10- 9. 5500* 5. 02 10- 5. 33X10 5. 90X10- 8.9)(10 Thermoelectricpower, S (microvolts/deg). 80 123 108 75 107 179 Thermal conductivity, K (cal/cm. see.

deg.) 1. 88 10- 1. 64x10 4. 22x10 1. 88 10- 3. 09x10 1. 41x10- oonductivity type N N N N Y N N Z, Figure of merit (T. C.) 1. 4X10- 2. 3X10- 1. 3X10 1. 3 10 1. 5X10- 6.1)(10- What is claimed is:

1. A thermoelectric material having the composition 7' 50 C(and 60 C. 2. A'thermoelectric material having the composition copper 1.000%, silver 78.000%, tellurium 0.924% and selenium 20.076%, the said material in particulate'form 3 "having been pressed at a pressure of at least 1000 p.s.i.

and heated at a temperaturebetween 75 C. to 500 C.

direct current to a body which is composed of the composition silver 70'to 85%, copper 0.005 to 2%, tellurium 0.005 to 1%, and selenium4.000 to 29.99%, which body has been pressed at a pressure of from 500 to 200,000 pounds per. square inch "from a powder form existing as particles of from 0.01 to 1,000 microns, and the said pressed material sintered at a temperature between 75 C. and 500 C. for atime of from 15 minutes to 2 hours. 7. Process for the manufacture of a thermoelectric body which comprises consolidating a mass of finely- 3. A thermoelectric material having the composition I copper 1.080%, silver 84.230%,'tellurium 0.650%, selenium l4'.040%, the said material in particulate form having been pressed at a pressure of at least 1000 p.s.i. and

' heated at a temperature between 75 C. to 500 C.

least 1000 p.s.i. and heated at a temperature between 75 C. to 500 C.

5. A thermoelectric material having the composition silver. 70. to 85 copper 0.005 to'2%, tellurium 0.005 to" 1% and selenium 4.000 to 29.99% with the addition of I a member of the class consisting of sulfur, silver, selenium, tellurium, copper, nickel, mercury, lead, cadmium, bismuth, antimony, thallium, gold, and manganese, the

said material in particulate form having been pressed at divided particles having the composition silver to copper 0.005 to, 2%, tellurium 0.005 to 1%, and selenium 4.000 to 29.99% by applying thereto a pressure of at least 500 pounds per square inch, and maintaining the said consolidated mass at a temperature of from 30 C. to a temperature which is 50 C. below the melting point. 1

References Cited in the file of this'patent UNITED STATES PA TENTS,

2,229,482 Telkes Jan. 21, 1941 2,289,152 Telkes July '7, 1942 2,397,756 Schwarz Apr. 2, 1946 2,597,752 Salisbury May 20, 1952. 2,902,528 Rosi Sept. 1, 1959 2,952,980 Douglas Sept. 20, 1960 FOREIGN PATENTS 836,944 Germany Apr. 17, 1952 

6. A PROCESS OF COOLING WHICH COMPRISES APPLYING A DIRECT CURRENT TO A BODY WHICH IS COMPOSED OF THE COMPOSITION SILVER 70 TO 85%, COPPER 0.005 TO 2%, TELLURIUM 0.005 TO 1%, AND SELENIUM 4.000 TO 29.99%, WHICH BODY HAS BEEN PRESSED AT A PRESSURE OF FROM 500 TO 200,000 POUNDS PER SQUARE INCH FROM A POWDER FORM EXISTING AS 